Redundant in vivo proteolytic activities of Escherichia coli lon and the ClpYQ (HslUV) protease

The ClpYQ (HslUV) ATP-dependent protease of Escherichia coli consists of an ATPase subunit closely related to the Clp ATPases and a protease component related to those found in the eukaryotic proteasome. We found that this pr otease has a substrate specificity overlapping that of the Lon protease, an other ATP-dependent protease in which a single subunit contains both the pr oteolytic active site and the ATPase. Lon is responsible for the degradatio n of the cell division inhibitor SulA; ion mutants are UV sensitive, due to the stabilization of SulA, ion mutants are also mucoid, due to the stabili zation of another Lon substrate, the positive regulator of capsule transcri ption, RcsA, The overproduction of ClpYQ suppresses both of these phenotype s, and the suppression of UV sensitivity is accompanied by a restoration of the rapid degradation of SulA Inactivation of the chromosomal copy of clpY or clpQ leads to further stabilization of SulA in a Eon mutant but not in lon(+) cells. While either ion, ion clpY, or ion clpQ mutants are UV sensit ive at low temperatures, at elevated temperatures the ion mutant loses its UV sensitivity, while the double mutants do not. Therefore, the degradation of SulA by ClpYQ at elevated temperatures is sufficient to lead to UV resi stance. Thus, a protease with a structure and an active site different from those of Lon is capable of recognizing and degrading two different Lon sub strates and appears to act as a backup for Lon under certain conditions.